Paper-sludge/phenolic composites were fabricated using novolac-type phenolic resin and paper-sludge. Phosphate flame retardants containing halogen (TCEP, TCPP, SFR), halogenated flame retardant (EBPED) and inorganic flame retardant (ATH, MH) were introduced into the paper-sludge/phenolic composite as flame retardants. The flame retardancy of the composite was estimated with the UL 94 test. Flexural tests were carried out to study the effect of flame retardants on the mechanical properties of the composites. Both phosphated flame retardants and EBPED showed a flame-retardant effect on the composite. However, inorganic flame retardants showed no flame inhibition in the combustion of the composite. All combined flame retardant systems used in this study reduced the flammability of the composite. Especially, aluminium trihydroxide, which showed no flame-retardant effect in the single system, reduced the flammability of the composite in the combined flame retardant system. The mechanical properties of the composites were reduced by the addition of phosphate flame retardants, but aluminium trihydroxide slightly decreased the flexural modulus of the composite.